Mask inspection systems disclosed for example in published US patent application 2003/0156280, US patent application 2003/0179369 and US patent application 2003/0148114, aim to increase throughput and sensitivity of inspection tools. Mask inspection systems may use deep ultraviolet (DUV) lasers, having a short wavelength of 266 nanometers (nm) for wafer inspection and 193 nm for mask inspection, for example, in order to increases resolution and sensitivity of defect detection where small size defects, smaller than 50 nm should be detected and marked as suspected defects by inspection tools.
Optical systems generate wavefront (WF) aberrations that may be characterized, for example, by a Shack-Hartmann wavefront sensor configured to measure localized slope of the wavefront using spot displacement in the sensor plane.
WF aberrations may cause false detections, and/or detection misses, in die-to-die or cell-to-cell comparisons, for example, where an inspection system is configured to compare two images, which are expected to be identical. Due to optical system's WF aberrations, inspection system may erroneously classify regular objects as suspected defects or miss classify suspected defects when such defects do exist.
WF aberrations reduce the optical system quality and hence reduce the ability to detect and characterize reliably small defects, <50 nm, using DUV lasers. Additionally, WF aberrations may vary between inspection tools' optical systems that hence may have different signal-to-noise (SNR) ratio that may reduce further the reliability of mask inspection tools.
Modern semiconductor process tools' optical systems, used in mask inspection and stepper tools, are inherently more sensitive to the optical system's WF aberrations due to the short wavelength DUV lasers used to detect nanoscale defects having less than 50 nm sizes.
It should be noted that while about a million suspected defect locations may be found per mask or wafer optical inspection, only a small portion of the suspected defect locations are true defects that would probably cause a yield reduction, and the bigger portion of the suspected defects are false detections that should be screened by inspection tools.
In view of the above, it would be desirable to provide an on-tool WF aberrations measurement systems and methods that will enable measuring and monitoring routinely WF aberrations of semiconductor process tools and further will allow compensating for the measured optical system's WF aberrations in order to increase process yields and reliability of inspection of semiconductor process tools.
The information included in this background section of the specification, including any reference cited herein and any description or discussion thereof, is included for technical reference purposes only and is not to be regarded as a subject matter by which the scope of the invention is to be bound.